 /*******************************************************************************
  * Copyright (c) 2003, 2006 IBM Corporation and others.
  * All rights reserved. This program and the accompanying materials
  * are made available under the terms of the Eclipse Public License v1.0
  * which accompanies this distribution, and is available at
  * http://www.eclipse.org/legal/epl-v10.html
  *
  * Contributors:
  * IBM Corporation - initial API and implementation
  *******************************************************************************/
 package org.eclipse.osgi.framework.internal.core;

 import java.net.URL ;
 import java.util.Enumeration ;
 import org.eclipse.osgi.framework.util.KeyedHashSet;

 /**
  * This class is used to optimize finding provided-packages for a bundle.
  * If the package cannot be found in a list of required bundles then this class
  * is used to cache a null package source so that the search does not need to
  * be done again.
  */
 public class NullPackageSource extends PackageSource {
     static KeyedHashSet sources;
     private NullPackageSource(String name) {
         super(name);
     }

     public SingleSourcePackage[] getSuppliers() {
         return null;
     }

     public boolean isNullSource() {
         return true;
     }

     public String toString() {
         return id + " -> null"; //$NON-NLS-1$
 }

     public Class loadClass(String name) {
         return null;
     }

     public URL getResource(String name) {
         return null;
     }

     public Enumeration getResources(String name) {
         return null;
     }

     public static synchronized NullPackageSource getNullPackageSource(String name) {
         if (sources == null)
             sources = new KeyedHashSet();
         NullPackageSource result = (NullPackageSource) sources.getByKey(name);
         if (result != null)
             return result;
         result = new NullPackageSource(name);
         sources.add(result);
         return result;
     }
 }

